A fuel cell is a power generating apparatus for producing electric current by electrochemical reaction of a fuel with an oxidizing agent. More specifically, a fuel cell is basically a galvanic energy conversion device that chemically converts hydrogen or a hydrocarbon fuel and an oxidant within catalytic confines to produce a DC electrical output. In one typical form of fuel cell, cathode material defines the passageways for the oxidant and anode material defines the passageways for the fuel. The electrolyte separates the cathode and anode materials. The fuel and oxidant, typically as gases, are then continuously passed through the cell passageways separated from one another. Unused fuel and oxidant discharged from the fuel cell removes the reaction products and heat generated in the cell.
This invention has direct applicability to cells employing a solid, doped oxide electrolyte. In the solid electrolyte fuel cell, or solid oxide fuel cell, hydrogen or a reformed high order hydrocarbon is used as the fuel, and oxygen or air is used as the oxidant, with the operating temperatures typically being between 700.degree. C. and 1100.degree. C. The hydrogen reaction on the anode (the negative electrode and hereinafter referred to as the fuel electrode) with oxide ions generates water with the release of electrons. The oxygen reaction on the cathode (the positive electrode and hereinafter referred to as the air electrode) with electrons effectively from oxide ions. Electrons flow from the fuel electrode through the appropriate external load of the air electrode, and the circuit is closed internally by transport of oxide ions through the electrolyte. The electrolyte, however, electrically isolates the air electrode and fuel electrode from one another. The reactions are represented as follows: ##STR1## Hydrocarbons can also be used as fuel, whereby carbon dioxide and water are typically produced in the overall reaction.
The electrolyte functions by isolating the fuel and oxidant gases from one another, while providing a medium allowing the ionic transfer and voltage buildup thereacross. The fuel and air electrodes provide paths for the internal movement of electrical current within the fuel cell to cell terminals, which also connect them with an external load. The voltage across a single cell is on the order of 0.7 volts maximum. Accordingly, multiple cells must be provided and placed in electrical series to obtain a useful voltage.
A series of electrical connection is accomplished between adjacent cells with an interconnect material which isolates the fuel and oxidant gases from one another, yet electronically connects the fuel electrode of one cell to the air electrode of an adjoining cell. As the active electrochemical generation of electricity takes place only across the electrolyte portions of the fuel cell, any interconnect separation between a fuel electrode and adjoining air electrode renders that part of the fuel cell electrically nonproductive. Accordingly, it is important that good interconnect bonding between fuel electrode and adjoining air electrodes is established during production and maintained under all operating conditions of the finished fuel cell.
Solid oxide fuel cells can be fabricated into a variety of geometries and cell designs. Typical designs include monolithic, planar and tubular. As well, a variety of fabrication methods can be used. The electrolyte, fuel electrode, air electrode, and current interconnection materials of a solid oxide fuel cell are generally considered as ceramic, which are densified and adjoining layers bonded to one another by sintering. Typical methods include tape casting, and cold pressing with co-sintering or separate sintering and bonding.
With tape casting, the individual powders are combined with an organic binder, an organic solvent, and a dispersant to a typical 50% oxide loading. The casting fluid is tape cast with a blade on a flat surface as a liquid, and thereafter allowed to dry into a thin plastic and flexible solid. Separate tapes are made for each of the air electrode, fuel electrode, electrolyte and interconnection. The tapes of the fuel electrode, electrolyte and air electrode are combined into a sandwich and formed into the desired geometry, with passages for gas flow. The interconnection tape is added or placed within the sandwich to join the cells together. The cell is next typically sintered in air in steps. For example, a slow, low temperatures heating to 823 K. over eight hours will remove the organic binder, solvent and dispersant, as each volatilizes and leaves the oxides. The cell is would typically be heated to 1823 K. in air and held for four hours, and thereafter cooled to room temperature.
With cold pressing and co-sintering, thin layers of the oxides are formed by cold pressing of the powders mixed with small amounts of binder, and formed into the desired geometry. The different layers are combined and isostatically pressed at room temperature. The fuel cell would then typically be heated slowly to 823 K. at 100 K. per hour, then at a rate of 300 K. per hour up to 1823 K., and held for four hours. It is thereafter cooled to room temperature. With cold pressing and separate sintering, individual layers of each component are sintered separately into desired geometries and thereafter bonded. This enables the sintering temperature for each component to be different to enable maximizing of a finished theoretical density. For example, the air electrode may be sintered at approximately 1723 K., the electrolyte and fuel electrode at 1674 K. and the interconnection at 1773 K. The separately sintered parts are machined to final configurations, and placed together into the shape of a fuel cell, with bonding slurry of the oxide having been painted on the mating surfaces. The cell is then heated to 1773 K. in air for 48 hours where the different materials are densified to their final density and bonded together as a completed fuel cell.
Regardless of the fabrication method, different properties of each of the air electrode, fuel electrode, electrolyte, and interconnect materials are preferably matched relative to one another. Yet, some important properties complete relative to one another such that a balance or compromise must be reached. For example, it is desirable that the thermal expansion/contraction properties of each material be very close to one another to avoid fracture between layers which could lead to destruction of the finished cell during fabrication and in operation. Further, it is important to maximize the electrical conductivity within each material. This is typically done by preselected stoichiometric doping of cations in the particular oxide material. However, typically different cations must be used each layer, which produces a large number of cations in the finished product which lead to electrochemical deleterious reactions between layers both during sintering and operation.
In considering the above criteria in selection of the various materials, the state-of-the-art solid oxide fuel cells use strontium doped lanthanum manganites as air electrodes (La.sub.1-x Sr.sub.x MnO.sub.3) (up to 10% Y.sub.2 O.sub.3 stabilized ZrO.sub.2 can be added to better match the thermal expansion), magnesium doped or strontium doped lanthanum chromites as interconnections (La.sub.1-y Mg.sub.y CrO.sub.3 or La.sub.1-y Sr.sub.y CrO.sub.3), nickel/zirconia as fuel electrodes (Ni-ZrO.sub.2), and yttria stabilized zirconia as the electrolyte ([1-z]Y.sub.2 O.sub.3 [z]ZrO.sub.2). Also, the state-of-the art tubular geometry utilizes a CaO stabilized ZrO.sub.2 porous support tube. These materials do not provide ideal combinations of thermal expansion and/or electrical properties, which makes fabrication difficult and can result in high thermal mechanical stresses. Further, present state-of-the-art powder synthesis and processing do not allow the lanthanum chromites to be sintered to high density in air below 1550.degree. C., which is preferred since the lanthanum manganite air electrodes need be sintered in air at or below these temperatures to produced an acceptable product. Additionally, the state-of-the-art construction has as many as nine cations (Zr, Y, La, Mn, Cr, Sr, Mg, Ni, and Ca) which can lead to potential chemical and electrochemical deleterious interaction between the materials in fabrication as well as operation.
Regarding thermal expansion properties, it is desirable to achieve the closest match possible. Constraints typically focus on the expansion properties of the electrolyte, requiring the other materials to be matched with those of the electrolyte. The thermal expansion of Y.sub.2 O.sub.3 stabilized ZrO.sub.2 varies with composition and is dependent upon temperature range. Thermal expansion generally decreases with increasing Y.sub.2 O.sub.3 above 8% (molar). The measured thermal expansion coefficients (298 to 1273 K.) for Y.sub.2 O.sub.3 stabilized ZrO.sub.2 electrolyte are as follows:
ZrO.sub.2 partially stabilized with Y.sub.2 O.sub.3 --10.0.times.10.sup.-6 /K.sup.1 (not the same on heating and cooling) EQU (0.906)ZrO.sub.2 -(0.094)Y.sub.2 O.sub.3 between 293 K. and 1273 K.--10.9.times.10.sup.6 /K.sup.2 EQU (0.92)ZrO.sub.2 -(0.08)Y.sub.2 O.sub.3 between 298 K. and 1273 K.--1.31.times.10.sup.-6 /K.sup.3
We measured a thermal expansion coefficient of 11.1.times.10.sup.-6 /K for (0.92)ZrO.sub.2 -(0.08)Y.sub.2 O.sub.3.
The reported thermal expansion coefficients for La.sub.1-y Mg.sub.y CrO.sub.3 for "y" ranging from 0.02 to 9.1.times.10.sup.-6 /K to 9.5.times.10.sup.-6 /K, respectively. Accordingly, the magnesium doped lanthanum chromite electrical interconnection material thermal expansion coefficient is significantly less than that of the typical yttria doped zirconia interconnection. The thermal expansion coefficient of La.sub.0.84 Sr.sub.0.16 CrO.sub.3 is 10.6.times.10.sup.-6 /K.
Use of lanthanum manganites and lanthanum chromites also creates problems during fabrication and/or during operation. For example, potential interaction of La with the electrolyte can lead to undesirable compounds, such as La.sub.2 Zr.sub.2 O.sub.7 which is an undesirable electrical insulator. Further, it is important that the lanthanum compositions be maintained so that La.sub.2 O.sub.3 does not form as a second phase. If the cation ratio of (La+Sr)/Cr is greater than one, or if Cr is depleted, the La.sub.2 O.sub.3 oxide can form. However, the best sintering of La.sub.1-x Xr.sub.x CrO.sub.3 results when this ratio is greater than (.gtoreq.1) which results in the undesirable formation of the La.sub.2 O.sub.3 oxide phase. If La.sub.2 O.sub.3 is present, even in small volumes, it will hydrolyze in the presence of water vapor at room temperature, forming La(OH).sub.3 and/or La.sub.2 O.sub.3.xH.sub.2 O. There is more than adequate water for hydration in an operating fuel cell in the oxidant air and in water being a byproduct of the reaction. A large volume change occurs upon the above hydration, resulting in fracture of the dense oxide and thus catastrophic failure of the cell.